It is known to polymerize olefins (a) using free radical initiators, (b) Ziegler-Natta catalysts based on titanium and vanadium transition metal compounds and (c) using metallocene based catalysts. These main types are associated with a certain type of polymer composition. Free radical polymerization gives fairy broad molecular weight distribution and extensive long chain branching. Titanium catalyst gives relatively narrow molecular weight distribution products containing significant levels of low molecular weight polymer having a high comonomer content. Finally metallocene catalysts generally give very narrow molecular weight and compositional distributions. Examples of the metallocene type are provided in EP 260999; WO EP 92/02803 and WO 92/14766. Page 19 of WO 92/14766 shows that in autoclaves generally a rise in the operating temperature takes place from the top of the reactor to the bottom as a result of the heat of polymerization. In cooled reactor systems such as tubular reactors that rise can be reduced.
It has been thought desirable to extend the range of polymer compositions producable by these metallocene and Ziegler-Natta catalyst systems and to provide a greater choice of molecular weight and compositional distribution.
Such attempts include blending different polymer compositions. EP 389611 (=WO 90/03414) uses narrow molecular weight distribution portions polymerized with metallocene catalysts and blends them to achieve desired molecular weight and compositional distributions. It is also known to arrange reactors in series and establish different polymerization conditions by varying temperature, monomer concentration and termination agent (such as hydrogen). It is also known to introduce initiators into an autoclave or tube at different positions or to use different catalyst compositions or concentrations (U.S. Pat. No. 3,536,693; U.S. Pat. No. 3,575,950; GB 251103).
GB 1314084 describes a multi-zone polymerization with vanadium and titanium based catalysts. The catalyst concentration exceeds 0.2.times.10.sup.-5 mol of transition metal per liter. Less than 30% of the monomer is converted in the first zone per pass to give a higher molecular weight material. Pressures are below 200 atmospheres. The monomer incorporation in the different polymer fractions would vary depending on the selection of catalysts in the first and second zone. In the examples generally the second zone uses a titanium based catalyst in addition to a vanadium catalyst. Titanium based catalyst tend to produce high molecular weight materials. A higher temperature is used in the second zone; hydrogen has to be added to limit the molecular weight. A broadened molecular weight distribution product is obtainable but at a high catalyst cost, which inclusion of a high comonomer, low molecular weight fraction and at low productivity.
DD 278476 and GB 1208120 also use Ziegler-Natta catalyst.
Finally EP 128045, EP 260130 and WO 93/13143 use simultaneously introduced different metallocene-based catalysts.
These prior art techniques for extending the range of available polymer compositions have drawbacks ranging from high capital cost (series reactors); deterioration in product quality (blending); difficulty of predictably obtaining the desired target polymer compositions and penalties in production rates of polymer.
It is amongst the aims of the invention to provide a relatively simple process for predictably broadening molecular weight distribution at low production cost and without producing a low molecular weight, high comonomer fraction.